Serious antisocial behavior, including criminal offending, is extremely costly to society. Rates of such behavior are highest during emerging adulthood. Antisocial behavior is especially high among emerging adults (EAs) with mental illness (MI); findings suggest the majority of EAs with MI will be arrested by age 25, most with multiple arrests, and for serious charges. Thus, there is a clear public health need for effective treatments to reduce serious antisocial behavior in EAs with MI. Astonishingly, there are no established interventions with evidence of efficacy to reduce serious antisocial behavior among EAs, with or without MI. Effective antisocial behavior interventions in adolescents address the comprehensive causes of that behavior. Similarly, this team has developed and completed research on a well-defined age-tailored intervention for EAs with MI and serious antisocial behavior that addresses the correlates of EA antisocial behavior, and provides MI treatment. The intervention is an adaptation of the well-established effective juvenile antisocial behavior intervention, Multisystemic Therapy (MST). MST-EA is a single source that targets the EA correlates of antisocial behavior, including gainful activity (school, work, housing, and positive relationships) and reduced substance use, in part by targeting the proximal mechanism of poor self-regulation. MST-EA also addresses these correlates through reducing MI symptoms. The investigative team has already established the safety, feasibility, and preliminary efficacy of thi type of intervention in a successfully completed community-based open trial (R34MH081374-01, PI: Davis). The proposed study will rigorously evaluate the effectiveness of MST- EA for reducing serious antisocial behavior. Specifically, 240 EAs with MI and recent arrests or release from justice facilities will be randomized to receive MST-EA or Treatment as Usual (TAU). Assessments will be completed at months 0, 2, 4, 6, 8, 12, and 16, with confirmation of outcome data using collateral reports and system records. The first aim will be to evaluate the effect over time of MST-EA for improving the ultimate outcome of treatment: reduced serious antisocial behavior. The second aim is to evaluate the effect of MST-EA on (a) the key proximal target of treatment (self-regulation) and (b) the proposed intermediate outcomes of treatment (gainful activity, substance use, and MI problems). The final aim will be to determine whether MST-EA's effect on the ultimate outcome is the result of its effect on the proximal target and intermediate outcomes of treatment. There is a current absence of any antisocial behavior treatments with demonstrated efficacy in this age group. The ultimate effect of the proposed research would be decreased antisocial behavior and other public health-related behaviors (MI symptoms, substance use, homelessness, unemployment) among one of the highest-risk population of individuals with MI. With an emphasis on treatment mechanisms and the near absence of MI research focused on EAs, this innovative research has high potential to advance the field.